1. Technical Field of the Invention
The present invention relates to a light source device comprising a bulb, a discharge medium sealed inside the bulb, and an electrode for exciting the discharge medium. Further, the present invention relates to a lighting device comprising the light source device, and to a liquid crystal display device comprising the lighting device.
2. Description of the Related Art
Recently, research on a light source device that does not use mercury (mercury-less type) as a lamp or light source device used for a back light device of a liquid crystal display device is actively progressing in addition to research on light source devices that use mercury. The mercury-less type light source device is preferable due to low fluctuation of light emission intensity along with time variation of temperature and in view of environmental concerns.
For example, a mercury-free type light source device shown in FIG. 43 is disclosed in Japanese Patent Application Laid-open Publication No. 5-29085. The light source device comprises a tube-shaped bulb 2 inside which a rare gas 1 is sealed, an internal electrode 3 disposed inside the bulb 2, and an external electrode 4 disposed outside the bulb 2. Further, a fluorescent layer 5 is formed on an inner surface of the bulb 2. The external electrode 4 has a strip-like shape extending parallel with an elongation direction of the bulb or a direction of an axis line L of the bulb 2. The external electrode is formed so as to Closely contact an outer surface of the bulb 2 by applying metal paste on the outer surface of the bulb 2, for example. The internal electrode 3 is electrically connected to a lighting circuit 6, whereas the external electrode 2 is grounded. When a voltage is applied between the internal electrode 3 and external electrode 4 by the lighting circuit 6, dielectric barrier discharge plasmanizes the rare gas so that it emits light.
Even if the external electrode 4 is formed by coating with the metal paste, the external electrode 4 cannot be completely in close contact with the outer surface of the bulb 3. In other words, as shown in FIG. 44, due to various causes, such as manufacturing error, vibration during operation, and the temperature status of the environment, a void or a slight gap 7 is inevitably generated between the external electrode 4 and the bulb 2. If the gap 7 exists, electric power cannot be supplied normally to the bulb 2. This causes instability in the light emission intensity. Further, a dielectric breakdown of an atmospheric gas tends to occur at the gap 7, and gas molecules ionized by the dielectric breakdown can damage the peripheral members. For example, if the atmospheric gas is air, the dielectric breakdown generates ozone which damages the peripheral members.
Even if a chemical method other than deposition is used, such as sputtering or adhesive, or a physical method such as mechanical pressing or a shrink tube is used, it is not possible to completely adhere the external electrode to the outer surface of the bulb. Therefore, a gap between the external electrode and the outer surface of the bulb will inevitably exist, causing unstable emission and dielectric breakdown of the atmospheric gas.
Further, in this type of light source device, it is important that time fluctuations in the light emission intensity, as perceived by humans, i.e., “flicker” be prevented. It is also important that the emission intensity be stabilized and dielectric breakdown of the atmospheric gas be prevented.